Compounds A and B are isomeric diketones of molecular formula C6H10O2. The 1H NMR spectrum of compound A contains two signals, both singlets, at δ 2.2 (six protons) and 2.8 (four protons). The 1H NMR spectrum of compound B contains two signals, one at 8 1.3 (triplet, six protons) and the other at 8 2.8 (quartet, four protons). What are the structures of compounds A and B?